1. Field of the Invention
The present invention relates to the neutralization of an electronic circuit when it is insufficiently powered. The invention is designed especially for circuits supplied with low power voltages. It can be applied especially in mobile telephony, contactless circuits and portable microcomputers.
During the buildup of the supply voltage in an electronic circuit, the functions of the circuit are generally neutralized and made inoperative, otherwise, their operation cannot be ensured. This is also the case when there is a big drop in the supply voltage or when the supply voltage is cut off.
To do this, devices have been developed to neutralize the working of an electronic circuit for supply voltage values below a threshold value. This threshold value is generally determined as a function of the value of the supply voltage and of the electronic circuit to be neutralized.
2. Discussion of the Related Art
Known neutralization devices conventionally comprise a control circuit and a means for inhibiting the working of the electronic circuit. The control circuit delivers the control signal that conditions the output of the inhibiting means. The shape of the control signal is shown in FIG. 1. The curve of the Power-On-Reset control signal referenced POR is superimposed on that of the supply voltage VCC so long as this supply voltage VCC remains below a threshold value Vs. Beyond this value, the voltage value of the control signal POR becomes zero.
The inhibiting means then converts the control signal into a binary signal. When the voltage of the control signal POR is zero, the binary signal has a high logic level. For any other value of the control signal, the binary signal coming from the inhibiting means is at a low level. Thus, for any value of supply voltage greater than the threshold voltage Vs, i.e. when the binary signal has a high level, the operation of the electronic circuit located downline with respect to the inhibiting means is permitted.
The inhibiting means may for example be a flip-flop circuit whose zero-setting input receives the control signal POR and whose "one-setting" input receives a signal for the activation of the electronic circuit. The binary signal coming from the inhibiting means then neutralizes the functions of the electronic circuits placed downline with respect to the inhibiting means when the binary signal delivered by this inhibiting means has a low level.
The control circuit is also the key element of the neutralizing device. FIG. 2 shows a known control circuit made for circuits powered by a voltage of 3 volts. It has a first means 1 to set the threshold value, connected by one input to a supply terminal Vcc and by its output to a ground terminal and by means of a resistor R1. This means 1 comes on when the supply voltage becomes greater than a threshold voltage of its own which is equal to the threshold value Vs.
In the example of FIG. 2, the means 1 consists of an N type transistor TN1 having its gate and drain connected to the supply terminal Vcc and its source connected to the source of a P type native transistor TP1. The drain of the transistor TP1 is connected to the resistor R1 and its gate is connected to the ground terminal And. It may be recalled that a native transistor is a transistor that has received no additional implantation in its conduction channel. Its conduction threshold voltage is greater in terms of absolute value than that of a P type enhanced transistor. In this example, this voltage is in the range of 1.3 volts in terms of absolute value.
The control circuit also includes an inverter I1 connected between the output of the means 1 and an output terminal OUT through which the control signal is delivered. A starting capacitor C1 is connected between the supply terminal Vcc and the output terminal OUT to obtain the output level expected during the beginning of the buildup of the supply voltage.
In this example, the threshold voltage proper to the means 1, representing the threshold value Vs, is equal to the sum of the values of the conduction thresholds of the transistors TN1 and TP1. In practice, it is slightly greater than this value for there must be a sufficient potential difference of the terminals of the resistor R1 to cause the inverter I1 to switch over.
Furthermore, two inverters I2 and I3 are cascade connected between the output of the inverter I1 and the output terminal OUT. These two inverters enable the regeneration of the signals present at the output of the inverter I1. At output of the control circuit, two possible levels are then obtained: a high level or a low level.
Preferably, the commutation ranges of the inverters I2 and I3 are offset with respect to each other in order to increase the stability of these two levels.
In operation, so long as the value of the supply voltage Vcc is below the sum of the conduction thresholds of the transistors TN1 and TP1, the value of the voltage at the input of the inverter I1 is zero. The output terminal OUT of the device then delivers a voltage equal to the supply voltage. As soon as the two transistors TN1 and TP1 become conductive, the output terminal OUT delivers a zero voltage.
In order that the neutralization device may work satisfactorily, there are conditions regarding the threshold voltage Vs. These conditions are taken into account when the control circuit is made. Indeed, the specification of the electronic circuit usually dictates a tolerance range for the value of the supply voltage. This range of tolerance is fixed by the customer. As a rule, it is +/-10% with respect to the nominal value of the supply voltage. The value of the supply voltage therefore ranges between a minimum value Vcc.sub.min and a maximum value Vcc.sub.max.
Furthermore, the threshold value Vs also has a range of uncertainty. Indeed, it is detected by a control circuit made out of components whose parameters may vary with the method of manufacture and the temperature. Hence, the threshold value Vs also varies between a minimum value Vs.sub.min and a maximum value Vs.sub.max.
A first condition in order that the neutralization device may work accurately is the following: Vs.sub.max &lt;Vcc.sub.min. Indeed, should Vcc=Vcc.sub.min and Vs=Vs.sub.max, it is obligatory that Vs should be smaller than Vcc, otherwise the threshold voltage Vs will never be reached by the control circuit and the neutralization device will permanently inhibit the working of the electronic circuit.
This first condition raises a problem if the value of the supply voltage is low. Indeed, the width of the range of uncertainty of the threshold value Vs not only is a function of the value of the supply voltage Vcc but also depends on the parameters of manufacture and of temperature. Although the width of the range of tolerance of the supply voltage diminishes as and when its nominal value diminishes, the width of the range of the threshold value remains almost identical. The above-mentioned first condition becomes increasingly difficult to fulfill.
Furthermore, it is also necessary to consider the fact that the components or groups of components of the electronic circuit to be neutralized have a functional limit Vf below which they no longer fulfill their role. Indeed, if we consider for example a path of current of the electronic circuit comprising two series-connected diodes, it is necessary that the threshold voltage Vs should be greater than the sum of the conduction threshold voltages of the two diodes in order that they may be on and may fulfill their conduction role. Consequently, it is necessary to have Vs.sub.min &gt;Vf.sub.max where Vf.sub.max is the maximum functional limit of the paths of the circuit.
For the electronic circuits supplied with voltages greater than or equal to 3 volts, the difference in voltage between the value of supply voltage Vcc.sub.min and the maximum functional limit to Vf.sub.max dictated by the electronic circuit is generally great enough for the threshold voltage Vs and its range of uncertainty to be interposed between these two limit values.
For circuits supplied by low voltages in the range of 1.8 volts, this difference is sometimes too small for the threshold voltage Vs to meet both conditions.
The most obvious solution would consist in reducing the size of the range of uncertainty of the threshold voltage Vs. For this purpose, it is necessary to resort to circuits of greater complexity and cost or to improve the performance characteristics of the method of manufacture. This is not always possible and would represent a heavy investment.
According to the approach of the invention, a neutralization device is made enabling the modification of the condition relating to the functional limit Vf.
The electronic circuit to be neutralized is divided into circuit portions combining one or more components on one and the same current path. Generally, one of these paths is said to be critical for it requires a minimum supply voltage Vf that is greater than the others for the efficient operation of the circuit portion pertaining thereto. This value of the supply voltage defines the value of the functional limit Vf of the circuit.
For one and the same circuit structure, there may sometimes be several potential critical paths owing to the variation of parameters inherent to the manufacturing method and to the operating temperature of the circuit.
In the control circuit of the invention, the potential critical paths of the electronic circuit are reproduced and arranged so as to determine a threshold voltage Vs that will be equal to the value of the maximum limit Vf of the electronic circuit for given conditions of temperature and manufacturing method. The condition Vs.sub.min &gt;Vf.sub.max will no longer have to be met since {character pullout}Vf.sub.max. In practice, this neutralization device and the electronic circuit are made on the same chip and are subjected to the same conditions of temperature and manufacturing method.